1. Field of the Invention
The invention relates to solid polyurethane powder coating compositions which contain uretdione groups and cure at low baking temperatures, to processes for preparing such compositions, and to their use for producing plastics, especially powder coatings, which crosslink to high-gloss or matt, light- and weather-stable coating films.
2. Description of the Background
Externally or internally blocked polyisocyanates which are solid at room temperature constitute valuable crosslinkers for thermally crosslinkable polyurethane (PU) powder coating compositions. For example, DE-A 27 35 497 describes PU powder coatings featuring outstanding weathering stability and thermal stability. The crosslinkers whose preparation is described in DE-A 27 12 931 are composed of isophorone diisocyanate which contains isocyanurate groups and is blocked with ε-caprolactam. Also known are polyisocyanates which contain urethane, biuret or urea groups and whose isocyanate groups are likewise blocked.
The disadvantage of these externally blocked systems lies in the elimination of the blocking agent during the thermal crosslinking reaction. Since the blocking agent may thus be emitted into the environment, it is necessary on environmental and occupational hygiene grounds to take special measures to clean the outgoing air and/or to recover the blocking agent. Moreover, the reactivity of the crosslinkers is low. Curing temperatures above 170° C. are required.
DE-A 3030539 and DE-A 3030572 describe processes for preparing polyaddition compounds which contain uretdione groups and whose terminal isocyanate groups are irreversibly blocked with monoalcohols or monoamines. A particular disadvantage are the chain-terminating constituents of the crosslinkers, which lead to low network densities in the PU powder coatings and thus to moderate solvent resistances.
Hydroxyl-terminated polyaddition compounds containing uretdione groups are subject matter of EP 0 669 353. On the basis of their functionality of two they exhibit improved resistance to solvents. A common feature of the powder coating compositions based on these polyisocyanates containing uretdione groups is that they do not emit any volatile compounds in the course of the curing reaction. However, at at least 180° C., the baking temperatures are high.
The use of amidines as catalysts in PU powder coating compositions is described in EP 803 524. Although these catalysts lead to a reduction in the curing temperature, they exhibit a marked yellowing, which is generally unwanted in the coatings field. The cause of this yellowing is probably the reactive nitrogen atoms in the amidines. These can react with atmospheric oxygen to give N-oxides, which are responsible for the discoloration.
EP 803 524 also mentions other catalysts which have been used to date for this purpose, but without showing any particular effect on the curing temperature. They include the organometallic catalysts known from polyurethane chemistry, such as dibutyltin dilaurate (DBTL), for example, or else tertiary amines, such as 1,4-diazabicyclo[2.2.2]octane (DABCO), for example.
WO 00/34355 claims catalysts based on metal acetylacetonates, e.g., zinc acetylacetonate. Such catalysts are in fact able to lower the curing temperature of polyurethane powder coating compositions containing uretdione groups, but as reaction products give primarily allophanates (M. Gedan-Smolka, F. Lehmann, D. Lehmann, “New catalysts for the low temperature curing of uretdione powder coatings” International Waterborne, High solids and Powder Coatings Symposium, New Orleans, Feb. 21-23, 2001). Allophanates are the reaction products of one mole of alcohol and two moles of isocyanate, whereas in the conventional urethane chemistry one mole of alcohol reacts with one mole of isocyanate. As the result of the unwanted formation of allophanates, therefore, isocyanate groups valuable both technically and economically are destroyed.